Monkeys were infected intranasally with Herpesvirus suis. After an incubation period of 7 to 13 days the animals became acutely ill and rapidly died. Clinical signs included salivation, incoordination, ataxia and epileptiform convulsions, but not pruritus. Histopathological changes were confined to the central nervous system and consisted of destruction of neurones with the formation of intranuclear inclusion bodies, gliosis and perivascular cuffing. Virus was isolated from the brain and spinal cord in the later stages of the illness but neutralising antibodies were not detected in serum. The distribution of lesions indicated direct spread of virus from the inoculation site along cranial nerves to the brain.